Imaging spectrometers that form high resolution images in a wide range of spectral bands are used in scientific, military and remote-sensing applications. Imaging spectrometers are used to provide an image and also a spectral analysis of the image in a selected wavelength band of interest. The spectrometer may image electromagnetic radiation that is passed through one or more slits onto one or more image sensors assembled at a focal plane of the spectrometer optics. In some spectrometers, such as double-pass spectrometers, the slits and the image sensors may be positioned adjacent one another at the focal plane.
One example of a spectrometer that is widely used for remote sensing applications is the Offner spectrometer. The Offner spectrometer form has a concentric structure that includes a slit, two concave minors and a diffraction grating disposed on a convex mirror positioned between the two concave mirrors. The Offner spectrometer provides very low distortion with few optical elements over small fields of view. However, the size of the structure becomes extremely large as the field of view increases. In addition, multiple spectrometers, each configured to cover a separate spectral band, as needed to cover wide wavelength ranges or separate spectral regions. As a result, these systems typically have large size, weight and power requirements due to multiple spectrometer optics and multiple image sensors.
U.S. Pat. No. 5,260,767, which is incorporated herein by reference in its entirety, describes examples of an all-reflective imaging spectrometer that uses a double pass reflective triplet form. The spectrometer includes a three-mirror anastigmat acting as its objective and a reflective triplet with a dispersive element providing the spectrometer collimator and imager. U.S. Pat. No. 6,122,051, incorporated herein by reference in its entirety, describes a single-band, multi-slit spectrometer that uses a two-dimensional detector array to enable simultaneous spectral analysis of several objects. Examples of a dual-band, dual-slit imaging spectrometer are described in U.S. Pat. No. 7,382,498, which is incorporated herein by reference in its entirety. According to U.S. Pat. No. 7,382,498, a two-channel spectrometer has a shared objective and a pair of slits at a common image plane. Each of the slits receives a portion of the output beam of the shared objective and is optimized for transmitting different wavelengths. A shared double-pass reflective triplet receives the output beams of the slits. The output of the reflective triplet is incident upon a beamsplitter, which sends a collimated first reflective triplet output of a first wavelength to a first dispersive element, and a collimated second reflective triplet output of a second wavelength to a second dispersive element. The outputs of the dispersive elements are directed back to the beamsplitter and the reflective triplet to imaging detectors positioned at two different locations of the common image plane.